[GVN] Assign new value number to calls reading memory, if there is no MemDep info.

[fhahn]

Summary

Currently we assign the same value number to two calls reading the same
memory location if we do not have MemoryDependence info. Without MemDep
Info we cannot guarantee that there is no store between the two calls, so we
have to assign a new number to the second call.

I added a new option EnableMemDep to enable/disable running
MemoryDependenceAnalysis and also renamed NoLoads to NoMemDepAnalysis to
be more explicit what it does. It also impacts calls that read memory,
so NoLoads is a bit confusing.

[efriedma]

PassManagerBuilder supports a boolean DisableGVNLoadPRE, but it looks like it's only hooked up to an actual user option if you're using LTO. You could add an option to GVN.cpp to override the default, to allow testing this codepath, I guess.

lib/Transforms/Scalar/GVN.cpp

396

Probably simpler to check MD && AA->onlyReadsMemory(C) here.

[fhahn]

Simplified, added EnableMemDep option and test.

[efriedma]

LGTM

lib/Transforms/Scalar/GVN.cpp

 }
 
 uint32_t GVN::ValueTable::lookupOrAddCall(CallInst *C) {
   if (AA->doesNotAccessMemory(C)) {
     Expression exp = createExpr(C);
     uint32_t e = assignExpNewValueNum(exp).first;
     valueNumbering[C] = e;
     return e;
   } else if (AA->onlyReadsMemory(C)) {
     Expression exp = createExpr(C);
     auto ValNum = assignExpNewValueNum(exp);
     if (ValNum.second) {
       valueNumbering[C] = ValNum.first;
       return ValNum.first;
     }
     if (!MD) {
-      uint32_t e = assignExpNewValueNum(exp).first;
-      valueNumbering[C] = e;
-      return e;
+      valueNumbering[C] = nextValueNumber;
+      return nextValueNumber++;
     }
 
     MemDepResult local_dep = MD->getDependency(C);
 
     if (!local_dep.isDef() && !local_dep.isNonLocal()) {
       valueNumbering[C] =  nextValueNumber;
       return nextValueNumber++;
     }